With increasing energy demands, declining petroleum reserves, and increasing environmental pressure, having clean, sustainable, reliable, and technically viable energy resources is one of the most significant challenges facing human society, industry, and the economy (Ragauskas et al., “The Path Forward for Biofuels and Biomaterials,” Science 311:484 (2006); Hoffert et al., “Advanced Technology Paths to Global Stability: Energy for a Greenhouse Planet,” Science 298:981 (2002)). Sustainable energy conversion and storage technologies, such as fuel cells, metal-air batteries, etc., attract enormous attention and have been intensively studied and developed given their potential for high energy-conversion efficiency and environmental advantages (Steele et al., “Materials for Fuel-cell Technologies,” Nature 414:345 (2001); Cheng et al., “Metal-air Batteries: From Oxygen Reduction Electrochemistry to Cathode Catalysts,” Chem. Soc. Rev. 41:2172 (2012); Yu et al., “Direct Oxidation Alkaline Fuel Cells: From Materials to Systems,” Energ. Environ. Sci. 5:5668 (2012); Varcoe et al., “Anion-exchange Membranes In Electrochemical Energy Systems,” Energ. Environ. Sci. 7:3135 (2014)). However, the sluggish oxygen reduction reaction (“ORR”), one of the half electrochemical reactions occurring at the cathode side, results in significant overpotential even at open circuit voltage operation (>250 mV), and largely limits the fuel cell's working efficiency and output power density (Varcoe et al., “Anion-exchange Membranes In Electrochemical Energy Systems,” Energ. Environ. Sci. 7:3135 (2014); Varcoe et al., “Prospects for Alkaline Anion-exchange Membranes In Low Temperature Fuel Cells,” Fuel Cells 5:187 (2005)). The noble metal-based catalysts, e.g. Pt, have been found to be the best ORR catalysts at low temperatures, but they suffer from several serious limitations including their limited reserves on the earth, high cost, and instability under the fuel cell operation environment (Cheng et al., “Metal-air Batteries: From Oxygen Reduction Electrochemistry to Cathode Catalysts,” Chem. Soc. Rev. 41:2172 (2012); Varcoe et al., “Prospects for Alkaline Anion-exchange Membranes In Low Temperature Fuel Cells,” Fuel Cells 5:187 (2005)). For direct alcohol fuel cells, noble metal-based ORR catalysts further have an alcohol poisoning issue, which is originated from the crossover of alcohol fuel from the anode to cathode (Li et al., “Nano-structured Pt—Fe/C as Cathode Catalyst in Direct Methanol Fuel Cell,” Electrochim. Acta 49:1045 (2004); Varcoe et al., “Anion-exchange Membranes In Electrochemical Energy Systems,” Energ. Environ. Sci. 7:3135 (2014)). With the alcohol crossover, electrochemical oxidation of alcohol competes with the ORR at the cathode and results in a mixing potential, thus reducing the fuel cell operating voltage and efficiency. Therefore, there is a clear and urgent need to seek for alternative noble metal-free or even metal-free cathode catalysts with high ORR activity, low economic cost, robust stability, and high tolerance towards alcohol fuels.
One strategy is to lower the noble metal content in the ORR catalysts. Adzic and co-workers reported the underpotential deposition method to fabricate a Pt monolayer on Au and Co (Brankovic et al., “Metal Monolayer Deposition by Replacement of Metal Adlayers on Electrode Surfaces,” Surf Sci. 474:L173 (2001); Adzic et al., “Platinum Monolayer Fuel Cell Electrocatalysts,” Top Catal. 46:249 (2007)). MPt (M=Fe, Co, etc.) alloys and core-shell (noble metal core) catalysts were also investigated to reduce the noble metal content (Li et al., “Nano-structured Pt—Fe/C as Cathode Catalyst in Direct Methanol Fuel Cell,” Electrochim. Acta 49:1045 (2004); Guo et al., “Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction,” Angewandte Chemie-International Edition 52:8526 (2013)). Another strategy is to replace Pt by transition metal macrocycle catalysts prepared through pyrolysis processes. Although these catalysts demonstrated competitive ORR activity relative to Pt, they suffer significant activity loss due to instability of the transition metal in the acid electrolyte (Wu et al., “Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction,” Accounts Chem. Res. 46:1878 (2013)). Recently, nitrogen doped carbon has been discovered as a metal-free ORR catalyst in a high pH medium. For example, metal-free nitrogen-doped carbon nanotubes were shown to have remarkable ORR activity and durability (Gong et al., “Nitrogen-doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction,” Science 323:760 (2009)). It has been reported that the carbon materials could become non-electron-neutral by incorporating N atoms into the graphitic framework. The electronic property tuning effect of N atoms can significantly change the charge density and spin density of carbon atoms; thus benefiting the adsorption of oxygen and subsequent reduction reaction on carbon (Gong et al., “Nitrogen-doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction,” Science 323:760 (2009); Liang et al., “Sulfur and Nitrogen Dual-doped Mesoporous Graphene Electrocatalyst for Oxygen Reduction with Synergistically Enhanced Performance,” Angewandte Chemie-International Edition 51:11496 (2012)). Besides nitrogen, other elements, such as S, B, and P, can also be incorporated into the carbon framework as a dopant or co-dopant with N to further enhance the ORR activity based on the synergistic effects between the two dopants (Wang et al., “Vertically Aligned BCN Nanotubes as Efficient Metal-free Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic Effect by Co-doping with Boron and Nitrogen,” Angewandte Chemie-International Edition 50:11756 (2011); Zhang et al., “A Metal-free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions,” Nat. Nanotechnol. 10:444 (2015); Yang et al., “Sulfur-doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction,” Acs Nano 6:205 (2012); Wu et al., “High-performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt,” Science 332:443 (2011); Jiang et al., “Nitrogen and Phosphorus Dual-doped Hierarchical Porous Carbon Foams as Efficient Metal-free Electrocatalysts for Oxygen Reduction Reactions,” Chem.-Eur. J. 20:3106 (2014)). Unfortunately, some undesired structures/phases (e.g., boron nitride) could be formed under high temperature annealing conditions, and can significantly lower the ORR activity of the catalysts (Liang et al., “Sulfur and Nitrogen Dual-doped Mesoporous Graphene Electrocatalyst for Oxygen Reduction with Synergistically Enhanced Performance,” Angewandte Chemie-International Edition 51:11496 (2012); Yang et al., “Boron-doped Carbon Nanotubes as Metal-free Electrocatalysts for the Oxygen Reduction Reaction,” Angewandte Chemie-International Edition 50:7132 (2011)). Thus, rational choice of dual-dopants and corresponding precursors play an important role in obtaining high ORR activity. However, few reports explain the criterion for choosing heteroatom precursors or investigate the precursor effect on ORR activity over heteroatom-doped nanocarbons.
Among nanocarbon materials, heteroatom-doped carbon nanotubes and graphene, one or two dimensional carbon networks with outstanding electric conductivity, have demonstrated excellent ORR performance, and have emerged as promising ORR catalyst candidates (Higgins et al., “Oxygen Reduction on Graphene-Carbon Nanotube Composites Doped Sequentially with Nitrogen and Sulfur,” ACS Catal. 4:2734 (2014); Zhang et al., “Substitutional Doping of Carbon Nanotubes with Heteroatoms and Their Chemical Applications,” Chemsuschem 7:1240 (2014); Gaoa et al., “One-step Pyrolytic Synthesis of Nitrogen and Sulfur Dual-doped Porous Carbon with High Catalytic Activity and Good Accessibility to Small Biomolecules,” ACS Appl. Mater. Inter. 6:19109 (2014)). However, their relatively low electrochemical surface area (“ECSA”) and randomly formed oxygen-inaccessible microspores may lead to internal diffusion issues in practical single fuel cell cathode applications (Varcoe et al., “Anion-exchange Membranes in Electrochemical Energy Systems,” Energ. Environ. Sci. 7:3135 (2014); Merle et al., “Anion Exchange Membranes for Alkaline Fuel Cells: A Review,” J. Membrane Sci. 377:1 (2011)). In comparison, mesoporous carbon nanostructures are more advantageous to serve as the cathode catalyst in a real fuel cell setting, because they feature not only high conductivity, but also well-ordered pore structure and tunable uniform mesopores (e.g., 4-20 nm) with a high ECSA (>1000 m2 g−1) (Jun et al., “Synthesis of New, Nanoporous Carbon with Hexagonally Ordered Mesostructure,” J. Am. Chem. Soc. 122:10712 (2000); Joo et al., “Ordered Nanoporous Arrays of Carbon Supporting High Dispersions of Platinum Nanoparticles,” Nature 412:169 (2001); Jun et al., “Synthesis of New, Nanoporous Carbon with Hexagonally Ordered Mesostructure,” J. Am. Chem. Soc. 122:10712 (2000)). Heteroatom (e.g., N) containing hydrocarbons are potentially attractive to serve as the carbon precursor; but some of them are very expensive, with prices comparable to noble metals (Liang et al., “Mesoporous Metal-Nitrogen-doped Carbon Electrocatalysts for Highly Efficient Oxygen Reduction Reaction,” J. Am. Chem. Soc. 135:16002 (2013); Xu et al., “Sulfur- and Nitrogen-doped, Ferrocene-derived Mesoporous Carbons with Efficient Electrochemical Reduction of Oxygen,” ACS Appl. Mater. Inter. 5:12594 (2013); Liu et al., “Nitrogen-doped Ordered Mesoporous Graphitic Arrays with High Electrocatalytic Activity for Oxygen Reduction,” Angewandte Chemie-International Edition 49:2565 (2010)). Thus, carbon precursor cost should also be taken into consideration for the catalyst preparation and utilization in fuel cells.
Heteroatom-doped carbons are rarely directly employed as cathode catalysts in H2/O2 fuel cells, which is probably due to membrane-electrode-assembly (“MEA”) fabrication issues (Varcoe et al., “Prospects for Alkaline Anion-exchange Membranes in Low Temperature Fuel Cells,” Fuel Cells 5:187 (2005); Jaouen et al., “Recent Advances in Non-precious Metal Catalysis for Oxygen-reduction Reaction in Polymer Electrolyte Fuel Cells,” Energ. Environ. Sci. 4:114 (2011)), related to underperforming electricity generation of H2/O2 fuel cells. For low temperature fuel cells directly fed with biomass-derived alcohols, such as ethanol and polyols, heteroatom-doped carbons are also attractive because they do not need a fuel reforming system and rigorous MEA fabrication process (Bambagioni et al., “Ethylene Glycol Electrooxidation on Smooth and Nanostructured Pd Electrodes in Alkaline Media,” Fuel Cells 10:582 (2010); Antolini, E., “Catalysts for Direct Ethanol Fuel Cells,” J. Power Sources 170:1 (2007)). In addition, the direct biorenewable alcohol fuel cells can simultaneously convert chemical energy into electrical energy and co-produce desirable biobased chemicals with the advantages of high efficiency, quiet operation and low CO2 emission (Yu et al., “Direct Oxidation Alkaline Fuel Cells: From Materials to Systems,” Energ. Environ. Sci. 5:5668 (2012); Li et al., “Nano-structured Pt—Fe/C as Cathode Catalyst in Direct Methanol Fuel Cells,” Electrochim. Acta 49:1045 (2004)). Therefore, such fuel cells are poised to take a significant role in the future energy landscape. The U.S. Department of Energy has identified in their top ten important chemicals ethanol and polyols (glycerol, sorbitol, etc.) (Bozell et al., “Technology Development for the Production of Biobased Products from Biorefinery Carbohydrates—The U.S. Department of Energy's ‘Top 10’ Revisited,” Green Chem. 12:539 (2010)), which can be economically derived from biomass, and can serve as building blocks for chemicals, fuels, and energy production in the future.
The present invention is directed to overcoming deficiencies in the art.